1. Field of the Invention
The present invention relates generally to telecommunications subscriber station data message processing, and more particularly to reducing idle current by selectively powering down the RF chain in the presence of a FOCC (forward control channel) data stream.
2. Description of the Related Art
Mobile telephone service using radio frequency (RF) transmission began in the United States in the late 1970""s. Initially, service was based on analog (FM) radio transmission for voice and Frequency Shift Keying (FSK) modulation, along with pre-assigned analog channels for control and signaling. ESS switching and standard trunking technology was utilized to provide access to and from the Public Switched Telephone Network (PSTN). This technology is commonly referred to as Advanced Analog Mobile Phone System (AMPS). An AMPS system consists of a Mobile Telephone Switching Office (MTSO) which can control multiple cell sites or base stations. The MTSO and cell sites communicate over standard voice trunks as well as dedicated control data links.
The United States Telecommunications Industry Association (TIA) has adopted a set of standards for migrating the current analog (AMPS) system to a dual mode operating environment in which a digital cellular network co-exists with the present analog service.
This standard subdivides existing air (radio channels) into six TDMA (Time Division Multiple Access) slots over which voice and control data will be transmitted. The xcfx80/4 DQPSK (Differential Quadrature Phase Shift Keying) modulation scheme is used in this scheme, commonly referred to as AMPS-D. The AMPS-D standard supports use of two time slots per frame to achieve full-rate coded voice transmission at approximately an 8 Kbps (kilobits per second) rate. AMPS-D uses control messages embedded within the traffic channels to perform call-related functions, this data being Forward Control Channel (FOCC) data, which is sent from a base station in frames containing data blocks that are repeated five consecutive times. Additionally, a code division multiple access standard has been adopted, which also includes and AMPS mode for backwards compatibility. The standard is promulgated by the Telecommunications Industry Association (TIA), and is commonly referred to at the Interim Standard 95 (IS-95).
As with all mobile telecommunication systems, handset power performance is of primary concern. The longer the handset can operate without the need for supplemental power or recharging, the more attractive the handset is to consumers. Within the framework of the current dual mode operating scheme employing an AMPS or AMPS-D analog standard, overall handset power performance may be optimized by decreasing FM idle current, or the current required when FM is in standby. FM idle current may be decreased given current FM architectures generally in three ways. First, FM idle current may decrease when overall RF subsystem performance is improved, such as by optimizing improving mixer/IF performance over previous designs. Alternately, other digital subsystems may be enhanced which decreases the need for idle current, including software modifications or power management techniques. One constraint of the current AMPS or AMPS-D system is that mobile phones utilizing these standards employ even or odd data stringing, whereby depending on whether a user has an even or odd phone number, the user will receive messages on either the A string or the B string. In other words, a phone powered down during a string in which the phone is to receive data cannot power up fast enough to receive and will miss that data. Under these conditions, the handset receiving the forward control channel data cannot be powered down
It is therefore an object of the current invention to provide a system for improving power consumption and overall power performance in a dual mode telephone handset employing the AMPS or AMPS-D standard including IS-95 as described above.
It is a further object of the current invention to provide power savings for a telephone handset receiving forward control channel (FOCC) data.
It is yet another object of the current invention to provide enhanced power performance for a telephone handset whereby existing known architecture and hardware are minimally affected.
According to the present invention, there is provided a system and method for reducing average idle current by decreasing the power in the RF stream when demodulation of the incoming signal and decoding of the forward control channel (FOCC) data stream is not necessary. The current inventive system takes advantage of the coding, transmission, and available a priori information about the FOCC to refrain from decoding the entire FOCC stream using FM slotting.
The present inventive system supports early completion of BCH decoding without the need to decode all five repeated 40 bit words received from the FOCC data stream. Rather than awaiting all five words of data in order to perform a full 3 of 5 evaluation, the inventive system disclosed herein begins an early evaluation to detect the validity of the received FOCC data. Initially the system fetches the initial data word. The system then assesses whether the first occurrence of the word is stored filler, or a control filler message. The presence of a control filler word may determined using a control filler mask. If the first occurrence of the word is stored filler, the system determines whether the word also passes CRC.
The land to mobile link provides control filler messages whenever no other message is required, and these control filler messages typically do not change for a given control channel. Observation of typical operational communication channels has shown that control filler messages occur over the FOCC channel approximately 80 per cent of the time. Moreover filler messages are discarded by the mobile as they convey no information.
The system stores the filler word if the CRC check is valid. Control filler messages may differ under certain conditions, but once a control filler message is received for a service in a particular geographic area, the form of the control filler message will be the same for subsequent transmissions. Thus the first time a control filler message is received by the handset, the handset must fully decode the entire control filler message. At each subsequent pass the contents of a control filler message are known and thus control filler messages may be discarded. Thus, the phone may sleep after the first filler message is properly received. If the first word is not a control filler the second occurrence of the data word is fetched, and the system evaluates whether the first fetched word is identical to the second fetched word, and whether a CRC pass occurs. This determination implicitly requires a BCH decode. If a CRC pass occurs the phone enters sleep mode. Sleep mode is a reduced power mode where receive processing is suspended until the next redetermined wake up period. The wake up period typically corresponds to the next paging slot. If either condition is not true, the system proceeds to fetch the third data word in the sequence.
The system then performs a majority after 3 evaluation, which evaluates the three words received to determine whether they are identical. If so, the system performs a BCH decode and, as with the first two evaluations, ceases further processing on the five data words and indicates sleep mode is appropriate.
Otherwise, the system fetches the fourth repeat of the data word. The system then assesses whether a bitwise majority of the four data words received match one another. Bitwise majority after 4 testing is similar to majority after 3 testing, but evaluates whether 4 bitwise majority has been reached and if so terminates decoding and sleeps. As with the other evaluations, this evaluation may be bypassed or disabled if desired.
Subsequent to this evaluation, assuming majority after 4 testing is negative, the fifth and final repeat of the data word is obtained. Further processing is provided to perform appropriate BCH decoding and storage of newly received control filler words, as well as setting BCH decode flags.
Each word block of a word received from the FOCC data stream begins with a 10 bit dotting sequence followed by an 11 bit sync word. The system typically declares a loss of synchronization if five successive sync words are incorrect. To further reduce active slot time, the system may erase the sync/dotting sequence if and only if the prior sync word is successfully decoded.
Sleep generation is the process of determining when the system, or a portion of the system, can be powered down under the arrangement described above. For each decode stream having A and B data, five slot cycles are supported correspond to the 5 repetitions of the transmitted 40 bit code word. The system must decode each slot to determine the slot is not a filler word.
When stream A slotting is mandated, and a one word slot determines the presence of remaining of the filler word. The system provides a power out condition in the presence of a control filler word until all five words have been received. The occurrence of an early decode assert in any of the first four word slots provides a power out condition, or sleep indication. The system then provides a power up indication during repeat word five such that the system will have sufficient warm up time to receive the next sync word if it is to be decoded, otherwise he data warm up timer goes high prior to receiving the next set of data to provide sufficient warm up time to receive data.
The system must decode each slot for a given stream A or B until the message can be decoded using one or more of the previously described early decode criterion otherwise all 5 repetitions of the data are decoded. In the stream B slotting condition, power down indication occurs on successful early decode of the message. The power up indication occur prior to the next sync if it is to be decoded or prior to the first repeat of the next message of stream B.